Modern Spain unique ability is add sticks to things... XD
There could be place for a modern Spain, yet the Hausgburg Hispanic Empire (with Conquistadores or Tercios) is clearly Exploration Age (discovery of new lands, territorial expansion). One could argue certain parts of Borbonic spain (Charles III, the Alphonses) and of Republican spain might fit the Industrialization/Progress topic of Modern age, and this could arge for inclusion of a different version of Spain. Key point for me to identify if it is viable is: ¿can we identify relevant uniques for these period that make an interesting modern age, modern-age mechanics Civ? (Guerrilla?, Royal Manufactories, perhaps?,...).
While it is true post-Hausgburg Spain did not have the same power than before, it is also true it still provided small sparks of genius that could be used for a modern Civ: a model for monarchic-liberal constitutions in 1812, support to US independence and negotiations with native americans, early prototypes of remote controlled boats, submarines, rotor-based A/C (autogyro) and even space suits...(and of course mops and some of the best lollypop versions - and even coca-cola origin is claimed in spanish kola-coca XD). Still having much other Modern european big-weight contenders (France, Britain, Prussia, Italy....) i'm afraid it would be relegated to DLC down the line if any.